LlamaIndex构建RAG时无法从文档中检索到目标信息的问题求助

我最近在尝试用LlamaIndex搭建RAG系统，遇到了一个特别困惑的问题：我用的是Ollama里的gemma聊天模型和nomic-embed-text嵌入模型，这套组合在LangChain里能正常检索到文档里的信息，给出我想要的回答，但换成LlamaIndex实现后，系统却一直说找不到相关内容，实在搞不懂哪里出问题了。

先给大家看看我在LangChain里能正常运行的代码，它能正确返回文档里的注册时间信息：

from langchain_ollama import OllamaEmbeddings
from langchain_community.llms import Ollama
from langchain.document_loaders import PyPDFLoader
from langchain.text_splitter import RecursiveCharacterTextSplitter
from langchain.vectorstores import FAISS 
from langchain.memory import ConversationBufferMemory
from langchain.chains import ConversationalRetrievalChain


model = Ollama(model = 'gemma', temperature = 0.1)
embedding = OllamaEmbeddings(model = 'nomic-embed-text')
raw_documents = PyPDFLoader(path+file).load()
text_splitter = RecursiveCharacterTextSplitter(chunk_size=1500, chunk_overlap=200) 
documents = text_splitter.split_documents(raw_documents)

db = FAISS.from_documents(documents, embedding)

memory = ConversationBufferMemory(memory_key='chat_history', return_messages = True)
query_engine = ConversationalRetrievalChain.from_llm(model, retriever=db.as_retriever(), memory=memory, verbose=True)
response = query_engine.run({'question':'when in the registration period'})

print(response)
# 输出结果：The registration period is between Jan to Feb.

上面这段代码完全没问题，能准确返回我要的答案。但换成LlamaIndex的实现后，就出问题了，代码如下：

from llama_index.llms.ollama import Ollama
from llama_index.embeddings.ollama import OllamaEmbedding
from llama_index.core import VectorStoreIndex, SimpleDirectoryReader, Settings
from llama_index.core.ingestion import IngestionPipeline
from llama_index.core.node_parser import TokenTextSplitter


Settings.llm  = Ollama(model="gemma", request_timeout=360.0)
Settings.embed_model = OllamaEmbedding(model_name="nomic-embed-text")


documents = SimpleDirectoryReader(path).load_data(file)
index = VectorStoreIndex.from_documents(documents)
query_engine = index.as_query_engine()

response  = query_engine.query("when in the registration period")

print(response)
# 输出结果：The provided text does not contain information regarding..., so I am unable to answer this query from the given context.

明明用的是相同的模型和文档，LlamaIndex却检索不到信息，我本来预期它能给出和LangChain一样的结果，现在实在找不到问题所在，有没有大佬能帮我排查一下？

备注：内容来源于stack exchange，提问作者HappyFish